Groundspeed is the magnitude of an aircraft's horizontal velocity relative to the ground. It determines travel time along a ground track, whereas aerodynamic lift and drag depend on motion relative to the air. Let and be the horizontal ground-relative and air-relative velocity vectors, and let be the horizontal wind velocity:For climbing or descending flight this horizontal convention distinguishes groundspeed from the magnitude of the full three-dimensional ground-relative velocity.[ref]
In an original horizontal-flight example, an airspeed of directed due north and a wind blowing east at give in east/north coordinates. Thus , with the track about east of north. Adding scalar speeds would wrongly give .
To keep a due-north track in the same wind, the aircraft must instead supply an air-relative westward component of . At the same airspeed, its northward component becomes , so the on-track groundspeed is smaller than the airspeed. The required wind correction angle is west of north. Heading and track are distinct directions.
Timed positions from a ground-based tracker estimate groundspeed, not airspeed. Inferring the latter also needs the wind velocity. Conversely, a Pitot-static tube does not measure travel over the ground. The same distinction explains why a headwind changes the ground range of a steady glider without changing its air-relative glide angle.
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